As the ON voltage of transistors decreases and elements become increasingly smaller, it is becoming more difficult to ignore the leak current of semiconductor devices, including gate arrays, since leak current causes a malfunction of the logic circuit and a read error of the storage element. Leak current also wastes the batteries of built-in equipment and decreases operation time due to the flow of unnecessary current.
In the design of such a logic cell, the configuration is determined so that the leak currents of P-channel and N-channel MOS transistors are equal values per unit width.
FIG. 16 is a table showing an example of the specifications of a standard type, low leak type and high-speed type MOS (Metal Oxide Semiconductor) transistors. The table shows the leak current, ON current, gate capacity for performance evaluation and junction capacitance per unit transistor width. These values are listed for the P-channel and N-channel of the standard type, low leak type and high-speed type MOS transistors respectively. The junction capacitance and gate capacitance are maintained at a constant level regardless the differences of these transistors.
The standard type MOS transistor shown in FIG. 16 is a transistor which is commonly used, and is the most frequently used transistor. The leak current of the standard type MOS transistor per unit width is 30 nA/μm for both the P-channel and N-channel, and the ON current per unit width is 350 and 800 μA/μm respectively. Whereas the low leak type MOS transistor is a transistor of which leak current per unit transistor width is suppressed (10 nA/μm), although the ON current per unit transistor width is suppressed (300 and 700 μA/μm). The high-speed type MOS transistor is a transistor of which ON current per unit transistor width is increased (400 and 900 μA/μm), although an increase of the leak current per unit transistor width is allowed (100 nA/μm).
For example, in order to save power of a semiconductor device, P-channel and N-channel MOS transistors in a logic cell are both changed from a standard type MOS transistor to a low leak type MOS transistor in the design stage. In other words, to construct a low leak type cell, a standard cell is constructed using standard type MOS transistors first, then design values are replaced with those of a low leak type P-channel and low leak type N-channel MOS transistors within a range where the drop in speed of the entire LSI is allowed, while maintaining the same transistor sizes.
In order to increase the speed of a semiconductor device, P-channel and N-channel transistors in the logic cell are both changed from standard MOS transistors to high-speed type MOS transistors in the design stage. In other words, to construct a high-speed type cell, standard cells are constructed using standard type MOS transistors first, then the design values are replaced with those of a high-speed type P channel and high-speed type N channel MOS transistors within a range where the leak power of the entire LSI is allowed, while maintaining the same transistor sizes.
The replacement of the design values, from the standard type to the low leak type MOS transistors, or from the standard type to the high-speed type MOS transistors, can be performed by changing the threshold voltage of the transistor. In an actual circuit, the threshold voltage can be controlled by adjusting the impurity implantation concentration. Patent Document 1 discloses a technology to control the threshold voltage of the MOS transistor by impurity concentration.
In a conventional logic cell construction method, the same type MOS transistors are used for P-channel and N-channel MOS transistors in a cell, regardless whether the input signal is high (hereafter H) or low (hereafter L). In other words, if the P-channel MOS transistor is a standard type, the N-channel MOS transistor is also a standard type, and if the P-channel MOS transistor is a high-speed type, the N-channel MOS transistor is also a high-speed type.
In a conventional low leak type cell, both P-channel and N-channel MOS transistors are low leak type MOS transistors, so leak current is low regardless whether the input signal is H or L. On the other hand, the ON current for driving the transistors in the next stage of the low leak type P-channel and N-channel MOS transistors is lower than the standard type, so the operation speed of the circuit is slow, regardless whether the input signal is H or L. Therefore in a conventional low leak type cell, leak current is suppressed by sacrificing speed.
In the case of a conventional high-speed type cell, both P-channel and N-channel MOS transistors are high-speed type MOS transistors, so operation is high-speed, regardless whether the input signal is H or L. On the other hand, the leak current of the high-speed type P-channel and N-channel MOS transistors is higher than the standard type, so the leak current is high, regardless whether the input signal is H or L. Therefore in a conventional high-speed type cell, the operation speed is increased by sacrificing the suppression of the leak current.
And generally speaking, when a logic circuit is designed, the leak currents per unit transistor width of P-channel and N-channel MOS transistors are set to equal values, as shown in FIG. 16. The leak current of the standard type MOS transistor is 30 nA/μm per unit transistor width for both the P-channel and N-channel. The leak current of the low leak type MOS transistor is 10 nA/μm for both [P- and N-channels]. The leak current of the high-speed type MOS transistor is 100 nA/μm for both [P- and N-channels].
A new demand is now emerging, however, to design a high-speed cell with low leaking by compositely applying low leak type, high-speed type and standard type MOS transistors for a P-channel and N-channel. But it is difficult to decide how to combine these different types of transistors, since the leak current, when different types of transistors are combined, cannot be easily estimated.
Also the mobility of a P-channel is low, so the transistor width of the P-channel is designed to be larger than the N-channel. Hence in the leak current of an entire LSI, leaking from a P-channel MOS transistor is greater if it is assumed that the switching probability is equal for the P-channel and N-channel MOS transistors. In other words, in terms of the manufacturing dispersion of P-channel and N-channel MOS transistors, the dispersion of P-channel MOS transistors has a greater influence on the leak current of an entire chip. As a result, chips which do not satisfy the product specifications are manufactured, and many defective products are produced.